1. Field of the Invention
The present invention relates to an indicator for the lock-up state of a lock-up type automatic transmission, and more particularly to an indicator for indicating whether the torque converter of the automatic transmission operates in a lock-up state or in a converter state.
2. Description of the Prior Art
Automatic transmission is provided with a torque converter in a power transmission system thereof so as to increase a torque from an engine. The torque converter has a pump impeller driven by an engine to rotate a working fluid within the torque converter wherein the rotation of the working fluid causes a turbine runner to rotate under the reaction of a stator, thereby multiplying the torque (torque converter state). Therefore, during the operation of the torque converter, there occurs a slip between the pump impeller and the turbine runner and therefore the automatic transmission provided with such a torque converter in its power transmission system is easy in operation but has a drawback of poor power transmission efficiency, leading to a poor fuel economy. To alleviate this drawback, there has been proposed a so-called torque converter with a direct clutch (also called a lock-up torque converter) wherein during a relatively high vehicle speed operation range when a torque variation of an engine does not create a problem a turbine runner is directly connected to a pump impeller (lock-up state), thereby eliminating a slip therebetween, and a lock-up type automatic transmission provided with a torque converter of this kind in a power transmission system thereof is already employed in some vehicles.
Where a torque converter with a direct clutch is caused to operate in a lock-up state whenever a vehicle speed exceeds a respective predetermined vehicle speed (lock-up vehicle speed) during operation in at least one of gear ratios, the lock-up ranges of such an automatic transmission are illustrated in FIG. 4. This Figure is a shift pattern diagram wherein V.sub.1, V.sub.2, and V.sub.3 denote predetermined lock-up vehicle speeds for a first gear ratio, for a second gear ratio, and for a third gear ratio, and A, B, and C denote lock-up ranges for the first gear ratio, the second gear ratio, and the third gear ratio, respectively. In the case of the automatic transmission wherein a lock-up clutch is engaged whenever the vehicle speed exceeds a lock-up vehicle speed during operation in each of the gear ratios, when the vehicle undergoes an automatic gear shifting operation with an accelerator pedal depressed to a great degree (a great opening degree of throttle), the torque converter remains in a lock-up state during gear shifting operation because the lock-up ranges A to C are adjoin one after another. However, if the gear shifting is effected with the torque converter remaining in the lock-up state, a torque variation is not absorbed by the torque converter, thus allowing a great shock to take place upon gear shifting.
Accordingly, the lock-up type automatic transmission of this kind is designed to suspend the lock-up state upon gear shifting even when the vehicle operates in the above-mentioned lock-up range. For this purpose, a gear shifting detector circuit is provided which issues a gear shifting signal indicative of a gear shifting and it is designed to temporalily suspend the lock-up state even during operation within the lock-up range while the gear shifting signal is being issued by this circuit.
In a vehicle installed with a lock-up type automatic transmission, it is advantageous for the purpose of enhancing market value to provide a lock-up indicator so as to have a driver to recognize whether the torque converter is within a converter state or lock-up state. Conventionally, a lock-up indicator has a pressure switch which is arranged to be responsive to a hydraulic pressure so as to provide an arrangement wherein the pressure switch turns on or off a lock-up indicator such as a monitor lamp thereby to inform a driver whether the torque converter operates within a lock-up state.
However, with such a lock-up indicator, the lock-up indication is temporarily turned off, although the vehicle operates within a lock-up range, in response to the above-mentioned suspension of a lock-up state during each gear shifting and subsequently turned on after the gear shifting has been completed, causing monitor lamp which usually constitutes the lock-up indication to turn off temporarily, interfering with the driver's view and causing the driver to take it as a sign that the lock-up indicator has gone out of order.